The present invention relates to a pressure regulator, particularly adapted to be installed in transport and distribution plants for natural gas.
It is well known that natural gas lifted at high pressure from reservoirs, is supplied to the user""s intake by proper transport and distribution plants.
These plants provide for delivering safely the combustible gas warranting delivery continuity and control of the delivery pressure value.
Said plants comprise a safety device consisting of a piloted pressure reducer of the Fail To Close (FTC) type, called also monitor, followed by a pressure regulator of the Fail To Open (FTO) type.
This configuration allows to supply gas to the user even when the FTO regulator fails.
Indeed in this condition the FTO regulator keeps open the gas inflow channel, assigning to the FTC monitor the task to regulate properly the gas delivery pressure, which is kept thereby just above the value set for the FTO regulator, but anyway within the safety limits.
If the monitor fails, it closes the high pressure gas inflow to the user so as to avoid accidents.
A known FTO pressure regulator is shown in FIG. 1, where one can see that it comprises a valve R having an inlet duct I for the high pressure gas and a delivery duct M for the low pressure gas, a regulation shutter O being arranged inbetween controlling the gas flow coming from the inlet duct I and therefore the gas delivery pressure.
The regulation shutter O is slidingly supported by a stem A fixed to an elastic membrane E belonging to the control head T in which the membrane defines a lower chamber Ci and an upper chamber Cs, the latter being connected to the delivery duct M.
The movement of the regulation shutter O is controlled through an auxiliary regulator Ra having an inlet way V connected to a duct F containing the gas under an intermediate pressure between high and low pressure, and an outlet way U communicating through a connection duct G with the lower chamber Ci of the control head T, feeding the reference pressure of regulator R.
The operation of the regulator provides that the shutter O is moved to change the gas delivery flow as a function of the difference between the gas delivery pressure and the gas reference pressure existing in the lower chamber Ci.
In order to calibrate the system, gas pressure in the lower chamber of the control head is set and the operator acts on the auxiliary regulator Ra so that the shutter remains substantially stationary when the delivery pressure is coincident with the desired pressure.
In this way the operation of the system provides that if the forces acting on the head are equal, the shutter remains stationary, while when the force exerted by the gas delivery pressure is lower than the desired value, the shutter opens increasing the high pressure gas flow and therefore increasing the gas delivery pressure until the two opposite pressures are again balanced.
Conversely an increase of the gas delivery pressure acts on the control head so as to impose to the regulation shutter a closure movement of the inlet duct for the high pressure gas with consequent decrease of the gas delivery pressure.
A first drawback of the above mentioned regulator R consists in that with a temperature variation the quantity of gas contained in the connection duct G between the auxiliary regulator Ra and the lower chamber Ci of the control head R, is varied thus causing variation of the gas delivery pressure.
As a matter of fact the duct G and the lower chamber Ci define a closed volume inside which gas is contained and it is well known that when temperature increases, also gas own pressure increases so as to cause a wrong opening of the regulation shutter O and therefore an increase of the gas delivery pressure.
The prior art in order to solve partially this problem, provides for installing an auxiliary tank S shown in dash lines constituting a volume of thermal dispersion connected to the connection duct G.
In order to further reduce the problems arising from the thermal drift, the prior art provides for isolating thermally both the connection duct G and the auxiliary tank S.
Another drawback of the described prior art consists in that pressure may increase to such an extent to become dangerous for the defined chamber.
In order to overcome such a drawback the prior art provides for installing a relief valve D shown in dash lines arranged on the connection duct G which is automatically actuated when gas inside the duct G exceeds a predetermined threshold pressure.
This solution however has a further drawback consisting in that gas is dispersed to atmosphere with fire and/or deflagration risks, jointly with the economic loss arising from the gas waste.
A further drawback of both prior art solutions, consists in that the thermal drift requires a frequent new calibration.
In order to solve the above mentioned drawbacks the prior art provides for using a different regulator of the FTO type shown in FIG. 2 which is different from the preceding one for the different feedback on the regulation valve R.
More particularly the system provides for placing a lamination valve Va provided with an inlet way Vi for the high pressure gas and two outlet ways U1 and U2: the first way U1 is connected to the upper chamber Cs of the regulation valve R1 and the second way U2 is connected to the inlet Pi of a pilot valve P. More particularly in the pilot valve P there is a first chamber B1 connected to the inlet way Pi and a second chamber B2 connected to a delivery duct M for the low pressure gas in which there is the plug of an interception shutter OP opening or closing the inlet way Pi.
The first chamber B1 has also an outlet way U3 communicating with the delivery duct M through a connection duct G1.
It is important to note that on the membrane E forces exerted by pressure of high pressure gas and by the gas contained in outlet V1 of the lamination valve Va are acting.
The operation of the FTO regulator provides that when downstream pressure rises above the desired limit, the pilot valve P closes the inlet way Pi preventing gas lamination by the lamination valve Va. This causes inflow of high pressure gas to the upper chamber Cs of the control head T and closes the inlet way I of the regulation valve R, so as to increase the downstream pressure.
Conversely with a decrease of the gas delivery pressure, shutter OP opens so as to allow inflow to the delivery duct M of gas coming from the inlet way Pi of the pilot valve P.
This allows gas lamination in the lamination valve Va and therefore decrease of pressure inside the upper chamber Cs of the control head. Consequently opening of the shutter O of the regulation valve R will occur, causing the gas delivery pressure to increase.
A drawback of the described prior art consists in that the regulation unit and the upstream regulator of the monitor FTC type are different from each other, so that they do not have interchangeable elements and cannot use common spare parts. This obliges the user to keep on stock a bigger quantity of spare parts and provide for a greater and differentiated training of the maintenance staff.
An object of the invention is to overcome said drawbacks.
More particularly a first object is to provide a pressure regulator having components that are interchangeable with the components of the upstream pressure regulator of the monitor FTC type.
Another object is to provide a pressure regulator keeping the gas delivery pressure constant when the operative temperatures of the regulation units are varying.
A further object is to provide a regulator in which the calibration is easy to be effected and must be carried out only once at the first start up stage.
A further object is to remove the fire and deflagration risks arising from an increase of the temperature of the entire regulation unit.
A last but not least object is to provide a regulator which is particularly simple to be used and installed.
The foregoing objects are attained by a gas pressure regulator that according to the main claim comprises:
at least a valve having an inlet duct for the high pressure gas Pa, at least a delivery duct for low pressure gas Pb and a shutter regulating the delivery flow of said gas slidingly supported by a stem fixed to an elastic membrane belonging to a control head of said valve and defining an upper chamber and a lower chamber;
at least a pilot valve for controlling the movement of said regulation shutter defining a first chamber with a first outlet way communicating with said delivery duct and a second chamber connected to said delivery duct in which the plug of an interception shutter adapted to open or close said inlet way is arranged;
said regulator being characterized in that said first chamber has at least a second outlet way communicating with said lower chamber and said upper chamber and first chamber are connected to each other through an auxiliary duct containing gas at an intermediate pressure Pm between said high pressure Pa and low pressure Pb.
Advantageously the regulator of the invention allows to convey in the delivery duct the excess gas at the high pressure developing inside the piloting duct.